A highly unusual solid‐state epitaxy‐induced phase transformation of Na4SnS4 ⋅ 14H2O (I) into Na4Sn2S6 ⋅ 5H2O (II) occurs at room temperature. Ab initio molecular dynamics (AIMD) simulations indicate an internal acid‐base reaction to form [SnS3SH]3− which condensates to [Sn2S6]4−. The reaction involves a complex sequence of O−H bond cleavage, S2− protonation, Sn−S bond formation and diffusion of various species while preserving the crystal morphology. In situ Raman and IR spectroscopy evidence the formation of [Sn2S6]4−. DFT calculations allowed assignment of all bands appearing during the transformation. X‐ray diffraction and in situ 1H NMR demonstrate a transformation within several days and yield a reaction turnover of ≈0.38 %/h. AIMD and experimental ionic conductivity data closely follow a Vogel‐Fulcher‐Tammann type T dependence with D(Na)=6×10−14 m2 s−1 at T=300 K with values increasing by three orders of magnitude from −20 to +25 °C.